What does George say he could do if Lennie was not around?

To answer this question, take a look at chapter one. Frustrated by Lennie's love of petting mice, George rants about the things that he could have if he did not have to take care of Lennie. First of all, he says:

I could get along so easy and so nice if I didn't have you on my tail. I could live so easy and maybe have a girl.

In other words, if Lennie was not around, George thinks that life would be easier. He could also have a girlfriend or a wife, because he would not have to worry about keeping tabs on Lennie.
Later in this chapter, George rants again about how easy life would be without Lennie. This time, he says:

God a'mighty, if I was alone I could live so easy. I could go get a job an' work, an' no trouble. No mess at all, and when the end of the month come I could take my fifty bucks and go into town and get whatever I want.

He also talks about how he could stay in a "cathouse" (a brothel) all night, play cards, drink whiskey and not have to worry about Lennie.
What we see here is that George has sacrificed a normal life in order to care for Lennie. Although he cares for Lennie very much and has no intention of walking out on him, George feels frustrated and isolated because of this responsibility.